Never mix brands, you will never be satisfied with what you get because the base pigments as well as drying agents are different in the different brands, they all have their own proprietary mixes, so you need to stay within the same brand to get the best results. Now take into account, it may be difficult to attain an exact match, the areas that you are not painting have been subject to environmental factors over the years and the new paint, might look like it when you mix it, but paint does not always dry exactly the color it is when wet, so there normally will be a small amount of difference when it is all said and done.
Last of all, good luck, mixing paints that are used on cars, trucks and boats can be some of the most difficult paints to mix properly in my experience. There is actually a lot that goes into paint to get the subtle hues, tones and colors that they do.
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